View Full Version : 12KP4A CRT good sub for 12JP4?


Phil Nelson
10-20-2016, 04:47 PM
I'm working on a Dumont RA-102 that has a 12JP4 picture tube. I have a chance to get a 12KP4A and I'm wondering if that's a good sub, as it seems to be from these data sheets:

http://frank.pocnet.net/sheets/127/1/12JP4.pdf

http://frank.pocnet.net/sheets/127/1/12KP4A.pdf

Nothing really wrong with my 12JP4, but this CRT is not aluminized and it doesn't use a bent gun, so it develops ion burn over time.

Unless I'm missing something, the 12KP4A is aluminized, eliminating the ion burn problem and possibly giving a little brighter picture. It also has an external aquadag coating, while the 12JP4 does not, but that's no big deal.

Regards,

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

Eric H
10-20-2016, 05:29 PM
It's an excellent sub, it's short like the JP4 and brighter with better contrast because it's aluminized.

Phil Nelson
10-20-2016, 05:43 PM
Okey-doke, I will pursue.

Thanks!

Phil Nelson

Eric H
10-20-2016, 06:48 PM
I would go after any 12KP4 that came along at a decent price, even if I didn't have an immediate use for it, they work great for replacing the 12LP4 as well.

reichsrundfu
10-20-2016, 07:16 PM
My RA103 Chatham has a 12KP4 and it's an awesome "daily driver". Direct sub. Go for it

Teorge

ChuckA
10-20-2016, 09:15 PM
I'm working on a Dumont RA-102 that has a 12JP4 picture tube. I have a chance to get a 12KP4A and I'm wondering if that's a good sub, as it seems to be from these data sheets:
http://frank.pocnet.net/sheets/127/1/12JP4.pdf
http://frank.pocnet.net/sheets/127/1/12KP4A.pdf
Nothing really wrong with my 12JP4, but this CRT is not aluminized and it doesn't use a bent gun, so it develops ion burn over time.
Unless I'm missing something, the 12KP4A is aluminized, eliminating the ion burn problem and possibly giving a little brighter picture. It also has an external aquadag coating, while the 12JP4 does not, but that's no big deal.
Regards,
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html


Phil,

Look at the physical dimensions and bulb shape of the 2 tubes, I found out the hard way they aren't completely interchangeable. I restored a RA-103 in a large Chinese decorated cabinet, the customer left me just the chassis as he was getting the cabinet restored. The 12JP4 was bad and the "spare" was also bad so I figured a 12LP4 would work OK, and it did as far as the chassis was concerned. When he tried to install the chassis with the new tube back in the cabinet we found that the difference in the 12LP4 bulb shape and the mask shape weren't compatible and the chassis wouldn't slide all the way into the cabinet. We had to have the depth of the mask cut down to fit the shape of the 12LP4, this allowed the chassis to slide all the way into the cabinet.

Chuck

Phil Nelson
10-20-2016, 11:29 PM
Hmm, good point. I ran into that issue once when using a rebuilt tube that had the right electronic characteristics but a slightly different bulb profile than the original.

Most of the dimensions look close enough, but now I see that the diameter at the 12KP4A screen face is 7/16" wider than the 12JP4. Of course, the dimensions given in the data sheets are plus-or-minus some fraction of an inch. I'll investigate whether there's enough wiggle room to accommodate it.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

ChuckA
10-21-2016, 07:45 PM
Hmm, good point. I ran into that issue once when using a rebuilt tube that had the right electronic characteristics but a slightly different bulb profile than the original.

Most of the dimensions look close enough, but now I see that the diameter at the 12KP4A screen face is 7/16" wider than the 12JP4. Of course, the dimensions given in the data sheets are plus-or-minus some fraction of an inch. I'll investigate whether there's enough wiggle room to accommodate it.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html


Phil,

It was the shape of the face that got me into the problem with the mask. Notice how much more the face of the 12J is curved compared to the 12K.

Chuck

Phil Nelson
10-21-2016, 11:50 PM
Aha. I just looked at the mask in my cabinet, and it's definitely shaped to fit the bulb-y profile. The price of this CRT is hard to resist, though, so I think I'll give it a try. If it doesn't work out in this set, I may be able to use it in another 12-inch TV.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

Ampico-kid
10-31-2016, 08:01 PM
Aha. I just looked at the mask in my cabinet, and it's definitely shaped to fit the bulb-y profile. The price of this CRT is hard to resist, though, so I think I'll give it a try. If it doesn't work out in this set, I may be able to use it in another 12-inch TV.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

Hey Phil....I subbed a 12KP4 for a 12JP4 in my Dumont Doghouse (RA103), and had no problems with fit or shape. Bob

StellarTV
11-01-2016, 12:24 PM
Phil, like Ampico-kid, I've subbed a 12KP4 in place of a 12JP4 minding that spherical face, and it turned out to fit nicely and look fine in a Dumont RA-103 once installed in the cabinet.

Phil Nelson
11-01-2016, 09:08 PM
Thanks. I did get the 12KP4A tube and it works fine in the chassis, with nice brightness & contrast. There's more electronic work to do, so I won't be trying to fit the chassis back into the cabinet for a while.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

Boobtubeman
11-01-2016, 09:52 PM
Someday, I'll get to my RA-103 :D

SR